Many electronic products need various amounts of memory to store information, e.g. data. One common type of high speed, low cost memory includes dynamic random access memory (DRAM) comprised of individual DRAM cells arranged in arrays. DRAM cells include an access transistor, e.g a metal oxide semiconducting field effect transistor (MOSFET), coupled to a capacitor cell. Another type of high speed, low cost memory includes floating gate memory cells. A conventional horizontal floating gate transistor structure includes a source region and a drain region separated by a channel region in a horizontal substrate. A floating gate is separated by a thin tunnel gate oxide. The structure is programmed by storing a charge on the floating gate. A control gate is separated from the floating gate by an intergate dielectric. A charge stored on the floating gate effects the conductivity of the cell when a read voltage potential is applied to the control gate. The state of cell can thus be determined by sensing a change in the device conductivity between the programmed and un-programmed states.
With successive generations of DRAM chips, an emphasis continues to be placed on increasing array density and maximizing chip real estate while minimizing the cost of manufacture. It is further desirable to increase array density with little or no modification of the DRAM optimized process flow.
A requirement exists for memory devices which need only be programmed once, as for instance to function as an electronic film in a camera. If the memory arrays have a very high density then they can store a large number of very high resolution images in a digital camera. If the memory is inexpensive then it can for instance replace the light sensitive films which are used to store images in conventional cameras. And, if the retention time is long then the memory can also be used in place of microfilm for archival storage.
Thus, there is a need for improved DRAM technology compatible write once read only memory. It is desirable that such write once read only memory be fabricated on a DRAM chip with little or no modification of the DRAM process flow. It is further desirable that such write once read only memory operate with lower programming voltages than that used by conventional flash memory cells, yet still hold sufficient charge to withstand the effects of parasitic capacitances and noise due to circuit operation.